Studies focus on the relationship of age to the effects of cardiac glycosides. In vitro studies detail the ability of ouabain to inhibit the Na-K ATPase pump, isolated from fresh cardiac tissue of fetal and mature sheep. The binding and dissociation of glycosides to and from the membrane system is examined also. In vitro studies in guinea pigs are performed to compare the effects of ouabain on the Na-K ATPase pump isolated from red blood cells with the pump isolated from myocardium. The pump inhibition studies are performed both by ultracentrifugation isolation of membrane systems and by rubidium uptake analysis. Clinical studies on the effects of digoxin are performed in infants and children under the treatment for congestive heart failure. Therapy is tailored to the individual patient by determining the pharmacokinetics of digoxin following the institution of loading therapy. The effects of digoxin on clinical status and myocardial function assessed noninvasively by echocardiography are compared with the effects of the drug on the Na-K ATPase system isolated from patients' red blood cells.